Sample or Random Security – A Security Model for Segment-Based Visual Cryptography

Abstract

In some scenarios, especially when visual cryptography [1] is used, the attacker has no access to an encryption oracle, and thus is not able to mount chosen-plaintext attacks. Based on the notion of real-or-random security under chosen-plaintext attacks (ROR-CPA) given by Bellare et al. [2], we propose the notion of sample-or-random security under ciphertext-only attacks (SOR-CO). We prove that the notion of SOR-CO is fundamentally weaker than the notion of ROR-CPA security and demonstrate the usefulness of our notion by applying it to segment-based visual cryptography [3]. An additional contribution of this paper is the construction of a new segment-based visual encryption scheme with noise based on work by Doberitz [4]. To our knowledge, this is the first visual encryption scheme which makes use of noise. We conjecture that it is secure in the sense of SOR-CO security if the key is not used too often and if the encryption schemes security parameters are chosen accordingly.